chore: migration guide for upgrading to latest 0.6.0 version

[bluntbrain]

Description

Updates the AgentKit 0.4.0 to 0.6.0 upgrade guide with:

• Added instructions for obtaining CDP_WALLET_SECRET from the CDP portal
• Created the guide in correct location python/coinbase-agentkit/docs/

Tests

Documentation-only changes, no functional tests required.

Checklist

• Added documentation to all relevant README.md files
• Added a changelog entry (N/A - docs only)

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[phdargen]

can be merged @CarsonRoscoe

[CarsonRoscoe]

I would love to merge this PR @bluntbrain , however our repo requires commit signing, and your first commit was not signed.

See here for instructions on setting up commit signing. Once setup, rebase against main to re-sign the commits.

Thank you!